40 
ELEMENTS OF GEOLOGY. 
tion of these operations, repeated as they are, year after 
year, and century after century; but I may suggest an 
explanation of the manner in which some micaceous sand¬ 
stones have originated, namely, those in which we see innu¬ 
merable thin layers of mica dividing layers of fine quartzose 
sand. I observed the same arrangement of materials in re¬ 
cent mud deposited in the estuary of La Roche St. Bernard 
in Brittany, at the mouth of the Loire. The surrounding 
rocks are of gneiss, which, by its waste, supplies the mud: 
when this dries at low water, it is found to consist of brown 
laminated clay, divided by thin seams of mica. The separa¬ 
tion of the mica in this case, or in that of micaceous sand¬ 
stones, may be thus understood. If we take a handful of 
quartzose sand, mixed with mica, and throw it into a clear 
running stream, we see the materials immediately sorted by 
the water, the grains of quartz falling almost directly to the 
bottom, while the plates of mica take a much longer time to 
reach the bottom, and are carried farther down the stream. 
At the first instant the water is turbid, but immediately af¬ 
ter the fiat surfaces of the plates of mica are seen all alone, 
reflecting a silvery light, as they descend slowly, to form a 
distinct micaceous lamina. The mica is the heavier mineral 
of the two; but it remains a longer time suspended in the 
fluid, owing to its greater extent of surface. It is easy, 
therefore, to perceive that where such mud is acted upon 
by a river or tidal current, the thin plates of mica will be 
carried farther, and not deposited in the same places as the 
grains of quartz; and since the force and velocity of the 
stream varies from time to time, layers of mica or of sand 
will be thrown down successively on the same area. 
Original Horizontality. —It is said generally that the upper 
and under surfaces of strata, or the “ planes of stratification,” 
are parallel. Although this is not strictly true, they make 
an approach to parallelism, for the same reason that sedi¬ 
ment is usually deposited at first in nearly horizontal lay¬ 
ers. Such an arrangement can by no means be attributed 
to an original evenness or horizontality in the bed of the 
sea: for it is ascertained that in those places where no mat¬ 
ter has been recently deposited, the bottom of the ocean is 
often as uneven as that of the dry land, having in like man¬ 
ner its hills, valleys, and ravines. Yet if the sea should go 
down, or be removed from near the mouth of a large river 
where a delta has been forming, we should see extensive 
plains of mud and sand laid dry, which, to the eye, would 
appear perfectly level, although, in reality, they would slope 
gently from the land towards the sea. 
